Assume a general linear model $y = X \beta + \epsilon$ with observations in an $n$-vector $y$, a $(n \times p)$-design matrix $X$ of rank $p$ for $p$ parameters in a $p$-vector $\beta$. A general linear hypothesis (GLH) about $q$ of these parameters ($q < p$) can be written as $\psi = C \beta$, where $C$ is a $(q \times p)$ matrix. An example for a GLH is the one-way ANOVA hypothesis where $C \beta = 0$ under the null.

The GLH-test uses a restricted model with design matrix $X_{r}$ where the $q$ parameters are set to 0, and the corresponding $q$ columns of $X$ are removed. The unrestricted model with design matrix $X_{u}$ makes no restrictions, and thus contains $q$ free parameters more - its parameters are a superset of those from the restricted model, and the columns of $X_{u}$ are a superset of those from $X_{r}$.

$P_{u} = X_{u}'(X_{u}'X_{u})^{-1} X'$ is the orthogonal projection onto subspace $V_{u}$ spanned by $X_{u}$, and analogously $P_{r}$ onto $V_{r}$. Then $V_{r} \subset V_{u}$. The parameter estimates of a model are $\hat{\beta} = X^{+} y = (X'X)^{-1} X' y$, the predictions are $\hat{y} = P y$, the residuals are $(I-P)y$, the sum of squared residuals SSE is $||e||^{2} = e'e = y'(I-P)y$, and the estimate for $\psi$ is $\hat{\psi} = C \hat{\beta}$. The difference $SSE_{r} - SSE_{u}$ is $y'(P_{u}-P_{r})y$. Now the $F$ test statistic for a GLH that is familiar (and understandable) to me is: $$ F = \frac{(SSE_{r} - SSE_{u}) / q}{\hat{\sigma}^{2}} = \frac{y' (P_{u} - P_{r}) y / q}{y^{t} (I - P_{u}) y / (n - p)} $$

There's an equivalent form that I don't yet understand: $$ F = \frac{(C \hat{\beta})' (C(X'X)^{-1}C')^{-1} (C \hat{\beta}) / q}{\hat{\sigma}^{2}} $$

As a start $$ \begin{array}{rcl} (C \hat{\beta})' (C(X'X)^{-1}C')^{-1} (C \hat{\beta}) &=& (C (X'X)^{-1} X' y)' (C(X'X)^{-1}C')^{-1} (C (X'X)^{-1} X' y) \\ ~ &=& y' X (X'X)^{-1} C' (C(X'X)^{-1}C')^{-1} C (X'X)^{-1} X' y \end{array} $$

• How do I see that $P_{u} - P_{r} = X (X'X)^{-1} C' (C(X'X)^{-1}C')^{-1} C (X'X)^{-1} X'$?
• What is the explanation for the numerator of this test statistic? - I can see that $C(X'X)^{-1}C'$ is $V(C \hat{\beta}) / \sigma^{2} = (\sigma^{2} C(X'X)^{-1}C') / \sigma^{2}$, but I can't put these pieces together.

Assume a general linear model $y = X \beta + \epsilon$ with observations in an $n$-vector $y$, a $(n \times p)$-design matrix $X$ of rank $p$ for $p$ parameters in a $p$-vector $\beta$. A general linear hypothesis (GLH) about $q$ of these parameters ($q < p$) can be written as $\psi = C \beta$, where $C$ is a $(q \times p)$ matrix. An example for a GLH is the one-way ANOVA hypothesis where $C \beta = 0$ under the null.

The GLH-test uses a restricted model with design matrix $X_{r}$ where the $q$ parameters are set to 0, and the corresponding $q$ columns of $X$ are removed. The unrestricted model with design matrix $X_{u}$ makes no restrictions, and thus contains $q$ free parameters more - its parameters are a superset of those from the restricted model, and the columns of $X_{u}$ are a superset of those from $X_{r}$.

$P_{u} = X_{u}'(X_{u}'X_{u})^{-1} X'$ is the orthogonal projection onto subspace $V_{u}$ spanned by $X_{u}$, and analogously $P_{r}$ onto $V_{r}$. Then $V_{r} \subset V_{u}$. The parameter estimates of a model are $\hat{\beta} = X^{+} y = (X'X)^{-1} X' y$, the predictions are $\hat{y} = P y$, the residuals are $(I-P)y$, the sum of squared residuals SSE is $||e||^{2} = e'e = y'(I-P)y$, and the estimate for $\psi$ is $\hat{\psi} = C \hat{\beta}$. The difference $SSE_{r} - SSE_{u}$ is $y'(P_{u}-P_{r})y$. Now the univariate $F$ test statistic for a GLH that is familiar (and understandable) to me is: $$ F = \frac{(SSE_{r} - SSE_{u}) / q}{\hat{\sigma}^{2}} = \frac{y' (P_{u} - P_{r}) y / q}{y^{t} (I - P_{u}) y / (n - p)} $$

There's an equivalent form that I don't yet understand: $$ F = \frac{(C \hat{\beta})' (C(X'X)^{-1}C')^{-1} (C \hat{\beta}) / q}{\hat{\sigma}^{2}} $$

As a start $$ \begin{array}{rcl} (C \hat{\beta})' (C(X'X)^{-1}C')^{-1} (C \hat{\beta}) &=& (C (X'X)^{-1} X' y)' (C(X'X)^{-1}C')^{-1} (C (X'X)^{-1} X' y) \\ ~ &=& y' X (X'X)^{-1} C' (C(X'X)^{-1}C')^{-1} C (X'X)^{-1} X' y \end{array} $$

• How do I see that $P_{u} - P_{r} = X (X'X)^{-1} C' (C(X'X)^{-1}C')^{-1} C (X'X)^{-1} X'$?
• What is the explanation for / motivation behind the numerator of the 2nd test statistic? - I can see that $C(X'X)^{-1}C'$ is $V(C \hat{\beta}) / \sigma^{2} = (\sigma^{2} C(X'X)^{-1}C') / \sigma^{2}$, but I can't put these pieces together.